Introduction

Soil, vegetation, and rock fragments form a thin surface layer on most land areas, but solid rock is always underneath. Rock underlies the sediments on the ocean floors as well. The deepest oil wells, which go down over 8 km, are drilled through rock similar to that at the surface. Some of the rock now out in the open was once buried several km inside the earth, and the material that makes up some volcanic rock probably rose in molten form from still greater depths, perhaps as much as 100 km down. These samples of rack from well below the earth's surface also turn out to be very much like rock that formed close to the surface.

Such direct observation tells us that the outer part of the earth, called its crust, is composed almost entirely of rock. However, the thickness of the crust is only 0.5 percent of the earth's 6400-km radius. There is no firsthand information about the rest of our planet, but its interior can be probed by indirect methods. After we have learned something about the structure of our planet and about the rocks that clothe it, we shall turn to the processes whose action has produced the landscapes around us.